1. Field of the Invention
The present invention relates to a method for exposing a pattern with an electron beam in a manufacturing process of a semiconductor integrated circuit, and more particularly to a method for drawing a desired pattern directly in a photoresist using a variably formed electron beam.
2. Description of the Related Art
As the high integration and fine pattern formation of a semiconductor integrated circuit have been developed in recent years, an electron beam drawing system is considered in place of a conventional exposure system using light in the photo-lithography technique. In the electron beam drawing system, a pattern is drawn directly on a photoresist film using an electron beam. Also, in the electron beam drawing system, there are two systems, a spot-shaped electron beam system and a variably formed electron beam system, depending upon the shape of the electron beam.
FIGS. 1A to 1D are diagrams illustrating a drawing method in the variably formed electron beam system. It is supposed that a desired exposure pattern 1 shown in FIG. 1A is exposed using the variably formed electron beam. An electron beam drawing apparatus has the maximum shot size of t.times.t which is the value peculiar to the apparatus, as shown in FIG. 1B. In this case, it is necessary to divide the exposure pattern 1 into rectangles having a size smaller than the maximum shot size, as shown in FIG. 1C. In other words, when the size of desired exposure pattern 1 is larger than the maximum shot size, the exposure pattern 1 can not be exposed at once. Therefore, the exposure pattern 1 is divided into rectangular patterns A, B, C, D, E, F, G, and H which have the size smaller than the maximum shot size, as shown in FIG. 1C. Then, the exposing process is performed by a rectangular electron beam with this size.
The pattern dividing process is performed by a rectangle dividing unit when the designed exposure pattern data is converted into a format peculiar to the electron beam exposure apparatus, or by a shot dividing unit in the electron beam drawing apparatus. Also, in the pattern dividing process, the exposure pattern is mechanically divided in the X axis direction and the Y axis direction.
Therefore, the rectangular pattern obtained through the pattern dividing process sometimes as an extremely small width than the maximum shot width t, as the rectangular pattern D shown in FIG. 1C.
Conventionally, even when such a small rectangular pattern D is generated, the exposure is performed with a reference light exposure quantity which has been previously fixedly determined. The reference light exposure quantity is applied to the other rectangular patterns A to C, and E to H. Also, the size of the used rectangular electron beam has been determined based on electron beam size calibration which is performed previously.
In this way, in the conventional electron beam exposure system using the variably formed electron beam, even if the small rectangular pattern is formed through the pattern dividing process, the drawing process is performed with the same exposure light quantity to all the rectangular patterns using the rectangular electron beam with the same beam size which has been previously calibrated in the size.
For this reason, the exposure light quantity is lack in the small rectangular pattern. As shown in FIG. 1D illustrating a cross section of the photoresist pattern when the photoresist pattern is cut along the line X--X in FIG. 1C, the resolution in the rectangular photoresist pattern 3D corresponding to the rectangular pattern D is lack after the drawing process. Thus, the drawing size precision of the electron beam is decreased consequently.
The size calibration of the rectangular electron beam will be described below. FIG. 2A shows a size measuring method (edge method) conventionally used for the size calibration of the variably formed electron beam. In this size measuring method, the incident rectangular electron beam 5 is scanned on a step mark 6 for calibration, and the reflected electrons 7 are detected by a reflection electron detector 8 to measure the size of the scanned electron beam.
At this time, as shown in FIGS. 2B to 2D, the detected original signal 9 is subjected the primary differentiation or the secondary differentiation to generate a signal 10 or a signal 11. Thus, the size calibration of the rectangular electron beam 5 is performed. It should be noted that the signal waveforms shown in FIGS. 2C and 2D are theoretical waveforms. The actual differentiated waveform has a broader waveform because of signal noise.
In case of this size calibrating method, because the number of incident electrons decreases as the size of the rectangular electron beam becomes small, the strength of the obtained reflection electron signal also decreases. As a result, the S/N ratio of the reflection electron signal becomes so small that it is difficult to precisely measure the size of the electron beam. Therefore, the size calibration precision is lack for the small pattern.
For this reason, in the actual size calibration, the rectangular electron beam having the size of equal to or more than 0.5 .mu.m for the size calibration. Then, the size calibration of the electron beam is performed based on a size curve extrapolated using a plurality of measured values. In this manner, because the size of the electron beam equal to or less than 0.5 .mu.m is determined based on the extrapolated calibration curve, the size precision decreases as the size of the electron beam become small.
The line width of an isolated photo-resist pattern is actually measured after the size of the rectangular electron beam is calibrated and then the pattern drawing process is performed to the photo-resist pattern with an exposure light quantity. As a result, nevertheless the size calibration of the electron beam is performed, the size of the electron beam abruptly becomes small from the size of 0.17 .mu.m.
In other words, when a small pattern was exposed, the size of the rectangular electron beam calibrated for the small pattern is smaller than a desired size of the electron beam for the small pattern. It is found that the exposure light quantity is lack if a small size pattern is exposed with the same exposure light quantity as the exposure light quantity when a larger size pattern is exposed.
In Japanese Laid Open Patent Disclosure (JP-A-Heisei 5-217870), a method is disclosed in which a proximity effect table is provided for every pattern size and the size of an electron beam is corrected based on proximity effect.
In recent years, the formation of a fine pattern having the size equal to or less than 0.20 .mu.m is required with the high integration and fine processing of a semiconductor device. Therefore, in this case, the size precision equal to or less than 0.02 .mu.m which is .+-.10% of the design size is required. For this reason, it is necessary and indispensable to avoid the size calibration lack which is the cause of the decrease of this size precision.